This invention relates to the texturing of 3-dimensional computer graphic images and in particular to a technique in which a mipmap chain of texture images is automatically generated during the process of texturing an image.
Texture mapping is a well-known technique in computer graphics and is used to apply bitmaps of texture image to objects being shaded to increase the realism of their representation. The texture images are bitmaps which are stored in memory.
During rendering of an image, if textures are sampled only one per screen pixel, aliasing will become a significant problem. FIG. 1 shows a texture receding into the distance with a single texture sample per pixel. As can be seen, the smaller samples start to show visible aliasing artefacts.
Mipmappinq is a known technique which is used to reduce the aliasing artefacts in texture mapping. Essentially, mipmapping involves storing filtered versions of the source data being represented, at different view points. These view points typically represent different distances to the object. Thus, a typical mipmap would consist of the original texture plus further copies at ½, ¼, ⅛, etc. resolutions. To produce these lower resolutions, each pixel is a filtered average value of e.g. four pixels from the next highest resolution mipmap. Pre and post filtering of the texture can further improve the quality of the rendered image by further reducing aliasing and ringing artefacts.
When mipmaps and texture maps are stored in memory, it is known to use a method called twiddling to ensure that spatially related data is stored together. This can greatly improve memory bandwidth utilisation by reducing the number of page breaks within a read of a portion of e.g. texture data. Further memory bandwidth requirements can be obtained by using systems known as tile based rendering systems. This is a technique in which an image to be rendered is broken down into a set of smaller rectangular areas or tiles. Each tile is processed individually for the objects which may be visible within it thereby reducing memory bandwidth requirements and therefore the size of any internal storage buffers. Lighting and reflections may be applied to rendered 3D objects by using a technique known as cubic environment mapping. This operates by, for example, first rendering a scene as it would be seen if looking outwards from the face of an object into an environment map texture. This texture is then mapped onto the face of the object.
Cubic environment maps which are sometimes referred to as cubic maps, are textures that contain image data representing scenes surrounding an object, as if the object were in the centre of cube. Each face of the cubic environment map covers a 90° field of view in the horizontal and vertical and there are six faces per cube map. Each face is perpendicular to the X/Y, Y/Z, or X/Z plane in 3D space. These maps are implemented as a series of texture objects. Applications can use static images for cubic environment mapping or they can render into the faces of the cube map to perform dynamic environment mapping.